1. Field of the Invention
This invention relates to x-ray diagnostic imaging contrast formulations for imaging the gastrointestinal (hereinafter sometimes referred to as GI) tract. The formulations described herein have enhanced utility as oral/rectal GI diagnostic contrast agents.
2. Reported Developments
It is a common medical practice to employ barium sulfate formulations to image the gastrointestinal tract of patients. Barium sulfate can be given either orally to visualize the stomach and upper GI tract or rectally to visualize the colon and lower GI tract. Barium sulfate is usually administered as a suspension that has limited stability even with the addition of stabilizers, it is often too opaque to visualize all segments of the GI tract, it often forms clumps that yield resultant radiopaque areas on x-ray films and has poor patient acceptability characteristics. Poor patient acceptability characteristics include palatability, patient discomfort during and after administration and constipation of the patient. Barium sulfate also shows poor affinity for coating the GI mucosa and consequently the patient is often needed to be manipulated or even rotated to ensure that the barium sulfate suspension coats the gastric mucosa. Nevertheless, segments of the GI tract are often either obscured or are not adequately coated to be visualized. It is not uncommon for the patient to undergo repeated examininations to achieve satisfactory imaging results.
The most serious failings of currently available formulations of barium sulfate are that they do not adequately coat the entire GI tract, requiring subsequent examination, and they pose safety concerns especially with patients that are suspected of having intestinal perforations or obstructions. Perforations of the upper GI tract or small bowel occurs with sufficient frequency that the risk of localized tissue injury is present. It is also not uncommon for orally administered barium sulfate to accumulate proximally to an obstruction in the bowel causing impaction that can lead to eventual perforation of the GI tract. In addition, aspiration of barium sulfate in the lungs may cause occlusion of the bronchioli and resultant pulmonary sequela.
Aqueous barium sulfate formulations are less constipating than non-aqueous barium sulfate formulations, however, they are often hypertonic, and consequently are irritating to the gastric mucosa.
Another class of compounds that have been investigated for imaging the GI tract are oil based emulsions of iodinated organic substances. Emulsions that have particle sizes of &lt;0.3 microns have been reported to image the small intestines of dogs but 50-70% of the oil based emulsions are reported to be absorbed from the intestine. Oily emulsions also appear to coat to some extent both the upper and lower segments of the GI tract as compared with non-oil based contrast agents. Oily emulsions are not usually contraindicated for patients with intestinal perforations or with intestinal obstructions. The major problems associated with these oil based emulsions is their tendency to be absorbed from the GI tract and the inherent toxicity that may be associated with the absorption of these agents. Emulsions such as PANTOPAQUE, i.e., ethyl iodophenylundecylate, adhere to the gastric mucosa, have low viscosity, low surface tension, are miscible with tissue fluids and exhibit good radiopacity. However, the emulsions of these organic iodinated substances suffer from their undesirable toxic effects.
Accordingly, there is a need to provide oral GI diagnostic x-ray contrast agents that enable the visualization of both the upper and lower GI tract following a single administration, that is safe and efficacious and is not contraindicated for GI tract perforations and/or obstructions. Such an agent should have excellent mucosal coating properties for both the upper and lower GI tract, i.e. it should have mucoadhesive or bioadhesive properties that enable the entire GI tract to be visualized. The object of the present invention is to provide a formulation that enhances the imaging of the GI tract that takes into consideration the physical-chemical properties of the imaging agents, surface stabilizers and viscosity modifiers. There is a need to provide in such a formulation primary surface stabilizers with appropriate mucoadhesive properties and secondary excipients that provide for a marked improvement in imaging quality as compared with existing products and formulations. The identification of surface active stabilizers With bioadhesive or mucoadhesive properties that enable the imaging of the entire GI tract has not been reported to date. This represents a difficult technical problem that requires the application of mucoadhesive technology to develop appropriate surface active agents that will enable the entire GI tract to be visualized. Bioadhesion is usually achieved by interaction of either a synthetic or natural polymeric substance With the mucosal membranes of the GI tract. Such technology has been employed to enhance drug delivery by decreasing the transit time of a drug substance in the GI tract and hence promote an opportunity for enhanced absorption. With regards to the development of safe and effective x-ray contrast agents for visualizing the GI tract, it is important to identify mucosal adhesives that coat the GI surfaces and visualize diseased or abnormal tissues. Highly charged carboxylated polyanions are good candidates for use as bioadhesives in the GI tract. See, for example: Park, K. and Robinson, J. R., Bioadhesion: Polymers and Platforms for Oral-Controlled Drug Delivery; Method to Study Bioadhesion. Int. J. Pharm., 19, 107 (1984). The formation of a bioadhesive bond between a polymeric substance and the mucosal lining of the GI tract can be visualized as a two step process, i.e., initial contact between the two surfaces and the formation of secondary bonds due to non-covalent interactions. Bioadhesives specific for the GI tract must interact with the mucus layer during attachment. Mucus, a general term for the heterogeneous secretion found on the epithelial surfaces of the GI tract, is made of the following components: glycoprotein macromolecules, inorganic salts, proteins, lipids and mucopolysaccharides. These glycoproteins typically consist of a protein core with carbohydrate side chains. This forms a network of mucus that is a continuous layer covering the GI tract. From a bioadhesive perspective, mucus consists of highly hydrated, crosslinked linear, flexible yet random coiled glycoprotein molecules with a net negative charge. Understanding the principles of bioadhesion is the basis for formulating an oral contrast x-ray agent for GI tract visualization. Bioadhesion accounts for the interaction between a biological surface and a biomaterial substance. As noted previously, bioadhesive agents are usually polymeric substances that adhere to tissues by ionic or covalent bonds or by physical attachment. Several theories of bioadhesion have been published including electronic, adsorption, wetting, diffusion and fracture theories. Bioadhesives bind to membrane surfaces and are retained for variable periods of time.
Crystalline x-ray contrast agents do not inherently adhere to the mucosal surfaces of the GI tract. It has now been discovered that crystalline x-ray contrast agents modified by the addition of surfactants, however, can be rendered so that they adsorb onto the GI mucosal surface. This is achieved by the use of mucoadhesive surfactants. The primary difficulty with previously reported mucoadhesive surfactants is that they do not interact effectively with both the particles and GI tract uniformly so that both the upper and lower GI tract can be visualized by a single agent during one examination. The surfactants used for this purpose must adsorb sufficiently to the different regions of the GI tract to enhance visualization by the contrast agent. In practice, surfactants tend to be adsorbed at some biological surfaces differentially than at others due to a variety of complex reasons. There is a need for contrast agents that are adsorbed sufficiently over the entire GI tract to allow adequate and uniform visualization of the different regions of the GI tract.
It has now been further discovered that crystalline x-ray contrast agents modified by the addition of mucoadhesive surfactants in combination with certain pharmaceutically acceptable clays further enhance the GI imaging characteristics of formulations and render the formulations more palatable to the patient and possess better suspension properties.
In accordance with the invention there is further provided a method for x-ray diagnostic imaging of the GI tract which comprises orally or rectally administering to the patient an effective amount contrast producing amount of the above-described x-ray contrast compositions.
A method for diagnostic imaging of the GI tract for use in medical procedures in accordance with this invention comprises orally or rectally administering to the mammalian patient in need of x-ray examination, an effective contrast producing amount of a composition of the present invention. After administration, at least a portion of the GI tract containing the administered composition is exposed to x-rays to produce an x-ray image pattern corresponding to the presence of the contrast agent, then the x-ray image is visualized and interpreted using techniques known in the art.